Recent innovations in boiler technologies have led to the introduction of modular boiler systems making use of several small boilers for applications where, in the past, a single, larger boiler may have been used. Such modular systems are often adaptable for changing uses over time when, for example, an addition may necessitate greater boiler capacity than that originally needed in a building.
One of the challenges with multi-stage systems such as modular systems is the need to meet changing heat loads over time in a stable and efficient manner. In a given system, different times of the day may require different amounts of heat production. For example, given a relatively simple example of a three boiler system, during setback periods (e.g. night), only one of the boilers may be needed to satisfy the heat load of the building. During a warmup period (e.g. early morning) following a setback period, all three boilers may be needed, while during ordinary operation (e.g. late morning), only two boilers may be needed, and during light ordinary operation (e.g. mid-afternoon) a single boiler may be sufficient.
Control of such boiler systems can be further complicated by the relative efficiencies of certain boilers. For example, FIG. 1 is an illustrative graph of efficiency data for an example commercial boiler. It can be seen that efficiency may improve as return water temperature drops, while efficiency may drop as the percentage of total output capacity increases. It should be noted that a minimum firing rate is also sometimes needed for stability and safety purposes. Meanwhile, difficulties can arise with return water temperatures at low firing rates, as explained by Pouchak, et al., in U.S. Pat. No. 6,694,927, which is incorporated herein by reference.
When a system operates with a relatively light heat load, the characteristics of the system and boilers can create difficulties or inefficiencies. Often, the built-in deadband of a system creates a delay between an increase in load and an increase in system capacity. For example, if all boilers are off and a call for heat occurs in a lightly loaded situation, the deadband typically causes the system to wait before turning on a first boiler. By the time the first boiler comes on, however, system temperatures may be relatively far from their setpoints, and the firing rate of the first boiler turned on will quickly ramp up. If the heat load is small, however, the load can be quickly met and the boiler turned off. This cycle is inefficient and may create undesired system temperature variations.